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Circle skirt maths - explained!

∏ = 3.141592653589793238462...

How many times in Maths class did/do you ever wonder when in the hell you'd actually need to know equations like this in real life? Well, to my dismay, it turns out that when you like to make stuff, Maths is kind of fundamental. Especially when it comes to making circle skirts. One of the easiest and quickest garments you can whip up on your machine, provided you can first figure out the radius of your waistline circumference... Ugh. Fear not, my mathematically challenged friends - we have figured it out for you. And made some pretty skirts along the way!

***Update - we made an app! Now you can leave the pesky sums to us and concentrate on the best part - sewing!***

Ready class? Then let's begin.

We're going back to GCSE Maths here guys. Remember pi?? That crazy 3 with endless decimal places is what we need to figure out the radius of our waistline circumferences. And why exactly do we need to know the radius? We need the radius in order to measure, mark out and cut the perfect little circle -that will be our waistline- onto the fabric. Without the radius, the only way to mark out the curve would be by shaping your measuring tape into a quarter circle on your folded piece of fabric (this ad hoc method I wouldn't even suggest to lazy stitchers as the results would inevitably be inaccurate, leading to a whole lot of time consuming fixing and tweaking and hair pulling and fabric wasted...).

For each type of circle skirt, be it full circle, half circle or quarter circle, the mathematical equation we need in order to find out the radius will be slightly different. Before we look at each one individually, we first need to know the foundation equation: Circumference (c) + two lots of seam allowance (3cm) ÷ ∏ (3.14) = diameter (radius × 2). We'd also like to point out now that we will be using metric measurements ie. centimetres, not inches. Nothing against Imperial, only that our brains simply can't cope with Imperial decimals, what with them being in eighths as opposed to tens...!

A note about seam allowance: seam allowance needs to be added on to both your waistline seam and side seams. For the waistline, we will be subtracting 1.5cm from your final radius measurement. For the side seams, we add 1.5cm for each raw edge to be seamed to your initial waistline circumference measurement: half and quarter circle skirts will have only one back seam so add 3cm to your waistline; a full circle skirt cut from 2 pieces will have 2 side seams so add 6cm altogether).

Full circle skirt

Let us begin by pointing out that it is unlikely you will be able to cut a whole circle skirt from a standard piece of 45" width fabric (unless making a miniskirt). The following diagram assumes that you will be cutting 2 semi-circles and joining them at the side seams, with your zipper inserted into one of those side seams - please see our invisible zipper tutorial when you get to that part!

by hand london - circle skirt maths

Example: Your waistline measures 66cm (equivalent of a 26" waist). You are making a full circle skirt from 2 semi circles so you need to factor in the 4 raw edges that will be your 2 side seams. 66cm (C) + 6cm (4SA) = 72cm. 72cm ÷ 3.14 =  23cm (diameter). (23cm ÷ 2) - 1.5cm (waistline seam allowance) = 10cm (radius).

Tip: cut a piece of string the length of your final radius measurement and holding one end at the corner, use it to accurately mark out your curve.

Half circle skirt

Our waistline measurement now becomes a semi-circle, so in order to find the radius with our little equation we need to double the waist measurement.

by hand london - circle skirt maths

Example: (66cm x 2) = 132cm (2C) + 3cm (2SA) ÷ 3.14 = 43cm (diameter). (43 ÷ 2) - 1.5cm (SA) = 20cm (radius).

Tip: before hemming your circle skirt it's a great idea to put it on a mannequin and leave it to "drop" overnight. Seeing as parts of a circle skirt hang on the bias, they'll need some time to stretch out naturally so you can then sew an even hem. If you don't give the fibres time to drop, you'll end up with a wavy, uneven hem line.

Quarter circle skirt

Just as we doubled the waistline as we halved the circle skirt, so we need to quadruple the waistline for a quarter circle skirt. The following diagram shows the quarter circle piece being cut from a single piece of fabric, no fold.

by hand london - circle skirt maths

Example: (66cm x 4) + 3cm (2SA) = 267cm ÷ 3.14 = 85cm (diameter). (85 ÷ 2) - 1.5cm (SA) = 41cm (radius).

Phew... Broken out in a bit of a mental sweat there guys, and words like radius and circle have lost all meaning. We hope this has fully explained the maths behind constructing circle skirts and their variations - if not, let us know and we'll do our best to clarify anything further. We leave you with the pretty fruits of our experiments...and one final tip: hemming a curve can be tricky; try our rolled hem tutorial for a quick, slick finish!

Over and out x

  • Elisalex de Castro Peake
  • appDIYsewingskirttutorial

Comments on this post ( 37 )

  • Apr 07, 2014

    Hello,

    I’ve done some circle skirt maths for drafting a circle skirt to attach to an existing bodice pattern…would be interested in hearing others’ feedback on my sums :)

    http://belgianseams.blogspot.be/2014/04/mustard-and-maths.html

    — Emily - Belgian Seams

  • Mar 19, 2014

    Hi Carmen, I’m afraid that unless your fabric is a striped circle (like a target), or you’re prepared to cut and match maaaaany panels, you’re not going to be able to get a circle skirt with stripes going all the way around… You’re much better off making a gathered skirt from a long rectangle so that the stripes all go in the same direction. You can check out our gathered skirt DIY here – http://byhandlondon.com/blogs/by-hand-london/11628445-diy-gathered-skirt

    Hope this helps!

    — Elisalex - By Hand London

  • Mar 19, 2014

    I have a veeery striped fabric that i want to go all the way around my skirt nicely but have no idea how to cut it! please help

    — Carmen Fernandez

  • Mar 10, 2014

    Hi Salwa – I’m afraid you’re going to have one hell of a time looking for jersey in a 230cm width! I’m pretty sure it simply doesn’t exist… You’re only solution for making a maxi full circle skirt is to cut it in multiple panels. You might be able to cut 4 quarter circles out of a 150cm width fabric – just remember to add on the seam allowances for the extra side seams! Hope this helps x

    — Elisalex - By Hand London

  • Mar 09, 2014

    Hiya,
    Great explanation and step by step. I want to make FULL maxi circle Skirt.
    I am unable to find fabric that wide in the UK. Any suggestions on how else I could make it. From waist to feet I measured 115cm so I would need 230cm wide fabric. And they only sell that width of fabric for curtains and furnishings. Most dress fabrics are 150cm wide And that would be waaay to short for what I need.
    I am wanting a heavy weight Soft Jersey in that width. Any tips on where to look? any online stores in the UK?
    Thank you

    — Salwa

  • Feb 27, 2014

    Hi Kerry – hmmmm that sounds like a bug in the app…. a 1/4 circle maxi skirt should fit nicely on a 60" width fabric. If I were you, go by the given radius and then measure out the length manually. Good luck (and we’ll get that bug sorted ASAP!)

    — Elisalex - By Hand London

  • Feb 25, 2014

    I love the look of the quarter circle maxi in the last photo. When I use the Circle Skirt App, it tells me that my skirt will not fit on a 60" piece of fabric. How did you achieve that skirt? I am a novice when it comes to clothing construction, so any insight you have would be marvelous. Thank you!

    — Kerry

  • Jul 28, 2013

    Hi Jessica,

    You do that because 1.5cm is your waist seam allowance. Taking that away from your radius means that you end up with the correct waist measurement once you’ve sewn on your waistband. Hope this makes sense!

    — byhandlondon

  • Sep 20, 2013

    Hi! I think this is a great tutorial, but I do have a question. For the half circle skirt. When I measure my waist I subtract 1.5 cm for the dream allowance and then I have to add 3 cm for the side allowances right? Thanks ahead of time! :)

    — The Nerdy Seamstress

  • Sep 20, 2013

    Hi! Yes, you add the 3cm for the side seam allowances, but when you mark out the curve of the waistline, you then take off 1.5cm from the radius for the waistline’s seam allowance. Hope this makes sense!

    — byhandlondon

  • Dec 17, 2012

    Please help me with measurements.I cannot figure it out..:-( I want to make a full circle plus half a circle belly dance skirt that fit’s around the hips. Hip measurement is 96cm.

    — Liz

  • Jan 15, 2013

    I was wondering if I want to add a half circle skirt to a bodice to create a dress…where do I stand with waist measurements? (my waist is 71cm but the bodice comes out a little bigger with ease?) Do I minus the 1.5 then add the 3?

    — LadyD

  • Jan 15, 2013

    I would measure out the bodice pieces and use that as your waistline measurement so it all matches up nicely. You could always shave down the side seams (presuming your bodice has side seams that it!) once you’d made it up to make it fit your waistline more snugly. Hope this helps!

    — byhandlondon

  • Jul 28, 2013

    Why do you subtract 1.5 from your radius measurement?

    — Jessica

  • Nov 10, 2012

    Excellent skirt maths lesson – got the old brain thinking – but unsure why you subtract 1.5cm seam allowance from waist line radius measurement in all cases. Could you e-mail me a explanation please.

    many thanks
    Wanda

    — Wanda Smith- Australia

  • Nov 10, 2012

    Thanks! The reason we subtract 1.5cm for the waistline radius is so that we automatically add 1.5cm seam allowance onto the waistline where the waistband will be sewed on. Hope this makes sense!

    — byhandlondon

  • Nov 25, 2012

    thank you so much, this makes SOOOOOO much more sense than any other circle skirt tutorial that i’ve dug up, especially since you’ve got it all written out in words (i have what is basically numberical dyslexia, so having WORDS helps so much).

    i’m just curious; what would the formula be for a 3/4 circle skirt?? would i multiply by 1.5??

    thx again!!

    — oonagh

  • Sep 28, 2012

    I am in love with your quarter circle skirt, and plan to make my own. I don’t have enough width in my fabric to cut it the length I want. Is it possible to cut it on the fold? I’m pretty sure I need to do some recalculations, I just don’t know what! Can you help?

    — craftylittlebugger

  • Nov 07, 2012

    I love this its so helpful and I love the full its a more playful style and to keep it from flying up I just sew a small weight in the hem as I go to keep it down on windy days. And I had my sister help me with the math bit because that is what they are learning in school right now she used it as extra credit and got an A.

    — Katt

  • Nov 07, 2012

    Love the trick of the weight in the hem! I remember when I was taking apart my Grandma’s old curtains to make into something else, discovering the hem weights… Light bulb moment! Glad to hear your sister got an A too! Who woulda thunk it?? Circle skirt tutorial helping out in Math class…!

    — byhandlondon

  • Sep 17, 2012

    You’re most welcome! We’ve got a “perfect waistband” tutorial coming up real soon, so stay tuned for all you need to know…

    — byhandlondon

  • Sep 25, 2012

    Great tutorial, thanks!

    — Chelsea

  • Sep 27, 2012

    for a full circle skirt the diameter assumed was 30cm so radius is 15 = 15-1.5= 13.5cms how did you get 14.5cm
    for a half circle skirt the diameter assumed was 42cms so radius is 21=21-1.5=19.5cms how did you get 22.5 cms
    for a quater circle the diameter assumed was 84 cms so radius is 42= 42-1.5= 40.5
    how did you get 4.5 cms
    also In all case you said to simply decrease by 1.5 cms whats the reason

    Please explain the above

    — sharon

  • Sep 27, 2012

    Thank you for bringing this to our attention!! You’re absolutely right – just goes to show my lack of mathematical talent! I will amend this immediately.

    — byhandlondon

  • Sep 09, 2012

    Aaah, excellent! I’m dying to try out a quarter circle skirt. Thanks for all the tips and for making thinks simple!

    — Marie

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